Electromagnetic focussing device for cathode ray tubes



1966 F. KRATOCHVIL ETAL 3,230,415

ELECTROMAGNETIC FOCUSSING DEVICE FOR CATHODE RAY TUBES Filed Aug. 31, 1962 2 Sheets-Sheet l IN VE N TORS Friedrich Krotochvil 8r Herbert Schlegel BY i Y E f g 2 ATTO RN EYS Jan. 18, 1966 F. KRATOCHVIL ETAL 3,230,415

ELECTROMAGNETIC FOCUSSING DEVICE FOR CATHODE RAY TUBES Filed Aug. 31, 1962 2 Sheets-Sheet 2 Fig 6 mvavrons 75 Friedrich Kroiochvil 8 Herbert Schlegel ATTORNEYS United States Patent 3,230,415 ELECTROMAGNETIC FOCUSSING DEVICE FOR CATHODE RAY TUBES Friedrich Kratochvil, Hannover-Ricldingen, and Herbert Schlegel, Havelse, Hannover, Germany, assignors to Telefunken Patentverwertungs-G.m.b.H., Ulm (Danube), Germany Filed Aug. 31, 1962, Ser. No. 222,259 Claims priority, application Germany, Mar. 24, 1962,

21,826; Apr. 18, 1962, T 21,988

11 Claims. (Cl. 31531) The present invention relates generally to cathode ray tubes, and, more particularly, to a magnetic focussing system for controlling the shape of the electron beam of a cathode ray tube.

Stretching or expanding the cross section of an electron beam of a cathode ray tube (CRT) of a television receiver or the like in order to form an activated phosphorous dot of light of a particular shape has been known to be accomplished by distorting the focussing field with the longitudinal direction of the stretched or deformed dot of light extending at right angles to the usually horizontally extending lines of the picture raster.

In a known magnetic focussing system of this type, which is disclosed in United States Patent No. 2,790,920, one or more permanent magnets of annular shape are provided on the neck of the tube and magnetic lines of force thereof are disposed in the direction of the electron beam. On the periphery of the magnets and at two diametrically opposed positions, pieces of ferromagnetic material are mounted which are initially unmagnetized. This provides a magnetic focussing device constructed to provide a dot of light of desired shape in order to attempt to eliminate the lines appearing on the raster.

In another known arrangement a coreless coil is wound around the neck of the tube and provides for additional magnetic focussing. This coil is in the shape of an oval to provide an elongated dot of light, and is disclosed in Wireless World, 1952, pages 219-221. However, due to the large stray field provided by this coil a large amount of electrical power is needed.

Another arrangement is also known which is disclosed in Austrian Patent No. 189,661 wherein a shaped dot of light is provided by means of two magnetic plugs diametrically arranged adjacent the neck of the tube, and which are magnetized in a direction at right angles to the electron beam. This type of arrangement can also be used in conjunction with electrostatic focussing and because of the simplicity of the arrangement it can be mounted subsequent to assembling the device. However, the magnetic plugs must be very accurately adjusted because the position of the magnets is extremely critical.

With these defects of the prior art in mind, it is a main object of the present invention to eliminate the disadvantages of the prior art arrangements and to provide a device wherein adjustment of the magnets of the device is not necessary.

Another object of the present invention is to provide an arrangement wherein the direction of magnetization of the magnets is parallel or oblique with respect to the electron beam.

Still another object of the invention is to provide a device of the character described wherein adjustment of the device can be provided by shifting and rotating the entire arrangement on the tube neck rather than the individual magnets with respect to each other.

Still a further object of the invention is to provide a device of the type described wherein only a small amount of power is needed.

These objects and others ancillary thereto are accomplished according to preferred embodiments of the invention wherein a focussing device for cathode ray tubes is provided which is disposed on the neck of the tube and wherein the direction of the magnetic field of the permanent magnets, or the electromagnets having an iron core, extends parallel to or obliquely with respect to the electron beam and wherein the magnets are provided with pole shoes directed toward the neck of the tube. With such an arrangement, alignment or focussing or adjustment of the magnets is not necessary, and instead a shifting and rotation of the entire arrangement on the neck of the tube is suificient. Because of the provision of pole shoes, small permanent magnets, or electromagnets having small current consumption, are sufiicient for eifective operation.

Additional objects and advantages of the present invention will become apparent upon consideration of the following description when taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a perspective view of one embodiment of the present invention illustrating the CRT.

FIGURE 2 is a front elevational view of the magnetic arrangement provided in FIGURE 1.

FIGURE 3 is a front elevational view of another embodiment of a magnetic arrangement for focussing the electron beam.

FIGURE 4 is a sectional view taken through the arrangement illustrated in FIGURE 3.

FIGURE 5 is a perspective view illustrating the entire magnetic arrangement of FIGURES 3 and 4 in a disassembled condition.

FIGURE 6 is an elevational view providing a development of the opening of a modification of the embodiment illustrated in FIGURES 3 through 5.

FIGURE 7 is a sectional view similar to FIGURE 4 illustrating a further modification of the gap structure.

FIGURE 8 is a circuit diagram illustrating the type of circuit which would be used to provide a focussing system using the present invention.

With more particular reference to the drawings, FIG- URE 1 illustrates a CRT 1 having a neck 2 in which an electron beam 3 is produced. This beam is focussed onto the picture screen 4 by means of a focussing arrangement (not shown). This beam 3 is deflected by means of deflection coils 5 in both vertical and horizontal directions across the picture screen in order to produce a picture raster.

In such cathode ray tubes it is desired that picture definition or sharpness of focus be the best possible, and in order to achieve this it is desirable to dimension the dots of light to be as small as possible in order to render the so-called black-white jumps essentially free of distortion. With a decrease of the light dot, dark stripes occur between the lines and the width of these dark stripes becomes larger as the number of lines become smaller and the picture screen becomes larger. It is true that these dark lines create less of a disturbance as the viewer moves a distance away from the screen and that the lines do not create a disturbance if the viewer goes beyond a certain distance from the picture screen; however, it is frequently desirable that these dark lines be eliminated.

In the arrangement illustrated in FIGURE 1, elimina tion of the dark lines is accomplished by shaping the cross section of an electron beam 3 to be substantially eliptical as shown at 6. In order to provide the electron beam 3 with an ellipitcal cross section, a. magnetic arrangement 7 is provided which includes a body or housing constructed of plastic and in which the cylindrical magnets 8 and 9 are mounted having their magnetic fields extending parallel to the axis of the tube.

The body 7' is constructed to be somewhat resilient and open at one end with an opening 19 in. the middle to fit around the neck of the tube so that the arrangement 7 can be mounted onto the neck of the tube in a simple manner even subsequent to assembly. The plastic body 7' is resilient and this is accomplished by providing an opening 14 therein between the magnets in addition to the bore which is to encircle the neck of the tube. Furthermore, the side of the body which is diametrically opposite the opening 14is provided with a slit which may have a substantially large width if desired and which provides the body with a general U-shape.

Since the width of the dark lines is dependent upon the spacing between lines and this is variable dependent upon the various picture screen dimensions which are utilized, the field strengths of magnets 8 and 9 must be variable 'so that the sa'm'e'r'n'agnet arrangement can be used for appliances having different screen dimensions. Accordingly, one, two, or three recesses 10' are provided on each side of the plastic body 7 into which the magnets may be insertedas necessary.

The magnet arrangement 7 is illustrated on an enlarged scale in FIGURE 2 and a brass strip 11 is injected into or cast into the plastic body 7 in order to increase the spring eifect mentioned above. In those devices which will be subjected to shocks or vibrations, it may also be advantageous to provide a screw or clamp connection as illustrated diagrammatically at 12 and which is provided at the slit side 10 of the body in order to clamp the arrangement in a fixed manner onto the neck of the tube. Metal sheets 13 of ferro-magnetic material are mounted on both sides of the magnets and these sheets are formed into the shape of pole shoes.

As illustrated in FIGURES 3 through 5, electromagnets 8' and 9' may be used instead of the permanent magnets 8 and 9 illustrated in FIGURE 2. These electromagnets are provided with iron core and pole shoes and in this construction the field strength can be easily varied. With this arrangement, the advantage is provided that the current supply to the electromagnets can be controlled by means of switch 31 (see FIGURE 8) which is mounted to the outer wall of the housing so that a viewer can, at will, stretch the round light dot into an elongatelight dot. This is partciularly advantageous, since when viewing from a greater distance the sharpness of the picture is improved, while when viewing from a position close to the screen, the suppression of the line structure is often desirable.

It is also possible to render the magnets themselves adjustable by using cylindrical magnets magnetized in the radial direction and which are disposed at right angles to the axis of the tube. These cylindrical magnets are provided with pole shoes which are disposed one after the other in the direction of the tube axis and respectively surround half the circumference of the cylindrically shaped magnets. These are fitted to the neck of the tube with their free end and by rotating or shifting the magnets the magnitude of the field can be varied.

With more particular reference to FIGURES 3 and 4, an embodiment is illustrated wherein two electromagnets are provided having a magnetic field which is dis posed parallel to the electron beam. These electromagnets are formed by a single coil 16 as shown in FIG- URE 4 and a single iron core 15. This core is provided with recesses 17 and 18 at two diametrically opposed sides in order to weaken or lessen. the effect of the magnetic field at these locations. Thus, strong magnetic fields are only provided at electromagnets 8' and 9' which correspond to the magnets 8 and 9 of FIGURES 1 and 2. Thus, the core portions 13 correspond to the pole shoes of FIGURE 2.

This embodiment of FIGURES 3 and 4 is simple and may be fastened to the neck of the tube in an easy manner and be adjusted with no difiiculty.- Furthermore, there is the advantageous result that external stray fields are almost entirely suppressed because of the very'good shielding. Because of this, when the means for supressing the line structure of the picture is activated, very good sharpness is provided in the picture, not only in the center, but also in the corners. Furthermore, due to the shielding from disturbing stray fields, image rotation or picture shift, which occurs in prior art systems due to the stray field, is substantially decreased. In an embodiment which was constructed and tested, the image rotation was less than half the image rotation of the known devices.

FIGURE 4 illustrates further details of the arrangement which is shown in elevational view in FIGURE 3. In FIGURE 3 the device is illustrated as being mounted on the neck 2 of the picture tube. The annular coil 16 is provided in the hollow core 15 and by means of the central opening 19 provides an axially directed magnetic field having lines 'of forceas" indicated at 20. This holiow core is generally annular in elevation, as illustrated in FIGURE 3 and is substantially closed except for recesses 17 and 18 and :air gap 21 which is provided in the inner surface which defines the inner opening 19 of the core. This air gap extends circumferentially about opening 19 and is only interrupted in the vicinity of the two recesses 17 and 18.

The construction of FIGURES 3 and 4 is illustrated in perspective view and in disassembled condition in FIG- URE 5 wherein the two halves 22 and 23 of the hollow core, and the coil itself, are illustrated. The annular coil 16 is inserted into one of the halves of the core 15. The two leads or contact ends of the annular coil 16 extend outwardly of the core and are connected to soldering point 24 which are mounted on the core by any suitable means. Then, the other half of the core 23 is placed upon the lower half 22. The two collar portions 25 of the core half 22 and the two collar portions 26 of the core half 23 together form or define the inner opening 19 which has a diameter substantially equal to the diameter ofv the neck of the tube.

The collar portions 25 together with the collar portions- 26 define the air gap 21 which extends about the circumference of the opening 19, and this air gap 21, which is illustrated more clearly in FIGURE 4, is interrupted at the recesses 17 and 18, as mentioned above. Therefore, the axially directed magnetic lines of force 20 are weakened in the vicinity of the recesses and therefore an axially oriented magnetic field is provided which is disturbed with respect to its radial symmetry, and the field is strongest at 8 and 9' and thus at the pole shoes 13.

A pair of tongues or extending flanges 27 and 28 are mounted on the lower half 22 of the core, and these are constructed of an insulating material and are provided in the recesses of the core half and oriented approximately in the axial direction. The tongues are arranged.

to have a spring effect and are arranged so that when the core is placed onto the neck ofthe tube the tongues will fit snugly against the neck 2 so that the focussing device will be provided with a firm seating on the neck of the picture tube. This seating is provided by merely which flows through the annular coil 16. The focussing unit is to be mounted on the neck of the tube in such a manner that the magnetic field is weakened at two locations which are disposed approximately vertically, one above the other, so that the light spot is deformed into an ellipse from its otherwise generally circular nature and the ellipse is disposed at right angles with respect to the direction of the lines of the picture raster. Therefore, the two recesses 17 and 18 have to be positioned approximately vertically one above the other, as shown in FIGURE 3. Then, if it is desired that the suppressing device for suppressing the line structure of the picture raster be rendered ineifective, all that need be done is to shut off the current flowing throughcoil 16.

If desired, and this is particularly advantageous when tubes having short necks are used, the additional focussing field may be placed in the proper location by providing the air gap at the edge of the opening 19 rather than in the center of the opening, as is the gap 21 of FIGURE 4. This modification is shown in FIGURE 7. The air gap 21 is formed in a side wall of core and thus is turned toward the picture screen, and the focussing field is brought closer to the deflection device of the receiver. Therefore, the efliciency of the focussing field is increased since the electrode system at which deformation of the electron beam is to take place, is disposed partially within the portion of the picture tube which is encompassed by the deflection device.

According to a further embodiment of the invention, the air gap which extends about the surface of opening 19 of the hollow core 15 and which is interrupted by the two recesses 17 and 18, is of spiral or helical shape within the inner opening 19. That is, the air gap extends between the two recesses from the lower face or side of the core 15 as seen in FIGURE 4 to the upper face or side. This is clearly shown in FIGURE 6 which is a development of the inner surface of opening 19 and the air gap 21" in this embodiment is also interrupted by the recesses 17 and 18 and extends between the two recesses obliquely from one edge of the core 15 to the other edge thereof.

One advantage of this embodiment is that the power required for focussing to deform the light spot is decreased with this embodiment as compared to others. The reason for this is that, as mentioned above, the coil generates a magnetic field which is directed substantially in the axial direction. The electron beam is also oriented in the direction of the axis of the neck of the tube, but it can not be influenced by an axially extending field running parallel to the electron beam. The change or control of the electron beam which provides the deformation of the light spot is performed by components of the magnetic lines of force which are at right angles with respect to the electron beam. By obliquely positioning this air gap 21", the component of the magnetic lines of force disposed at right angles to the electron beam is increased, and in this manner the efiiciency of the focussing device is increased and the electrical deflection power which is required is decreased.

A circuit diagram of a typical circuit for the focussing system according to the present invention is illustrated in FIGURE 8. The entire focussing system is indicated by the dashed line 29 and includes the annular coil 16 and a resistor 30 which has a negative temperature coeflicient to compensate for the influence of temperature fluctuations in the coil. The focussing device for suppressing the line structure of the picture raster is connected to the positive terminal of the operating voltage source by means of a switch 31 and a resistor 32 which provides the correct D.C. value. The video signal is applied to the cathode of picture tube 1 by means of a line 33 in a known manner. The control electrode 34 of the picture tube 1 is directly connected to a resistor 35 disposed between the abovementioned positive terminal and ground in a known manner. The brightness of the picture can be adjusted by changing the resistance of this resistor.

When the device for suppressing the line structure is switched olf by means of the switch 31, a switch 36 which is ganged thereto is simultaneously actuated and places the focussing electrode 37 at a different potential. This different potential provides the optimum focussing voltage for the picture tube, and this voltage is different depending upon whether the line suppressing device is on or otf. This optimal focussing voltage changes when current is provided through coil 16 from a previously positive voltage to a negative voltage, the voltage being measured with respect to the cathode. If current is not provided through coil 16, the focussing electrode 37 is connected to the positive terminal of the operating voltage via a resistor 38 which may be used to control the sharpness of the picture and also by means of a dividing resistor 39. When current is provided in coil 16, the focussing electrode 37 is then connected to ground and has a strong negative potential with respect to the cathode of the picture tube. Due to this circuit arrangement, the focussing voltage is always set for optimum conditions both when the device for suppressing the line structure is being used by being switched into operation and when it is switched into the otf condition.

The sliding contact 40 is brought into operation for a brief period when the contact of the switch 31 is switched from one position to the other. This contact places the control electrode 34 of the picture tube at ground potential for a short time and because of this the picture tube is so controlled that it is dark for a short period of time. Therefore, the minor rotation of the picture or image which occurs when a current through coil 16 is switched on or off, and which is caused by the stray field of the focussing current, cannot be seen by the viewer.

In an embodiment of the present invention, there has been used magnets providing a magnetic field of oersteds in the axis of a 23 cathode ray tube giving (in the center of the picture) an elliptical light spot, the dimensions of which are 1.3 mm. in vertical and 0.83 mm. in horizontal direct-ion (625 lines) compared with a spot of 1 mm. diameter without using magnets.

It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

What is claimed is:

1. A magnetic focussing device for a CRT having a screen and a neck for elongating the dots of light provided when the electron beam bombards the screen, said device comprising a body of plastic material arranged to be mounted on the neck of a CRT, two magnet systems mounted in said body and arranged to be diametrically opposite each other with respect to a tube neck with the magnetic fields there-of disposed in a position which is other than at right angles with repsect to the electron beam of the tube, said magnet systems having pole shoes arranged so that when the device is on a tube the shoes are directed toward the tube neck, said pole shoes each extending over only a small portion of the circumference of the neck of such tube to form within the tube neck a magnetic field which is radially unsymmetrical and which is substantially parallel to the electron beam and to elongate the luminous spots on the screen in a direction which is at right angles to the screen lines.

2. A device as defined in claim 1 wherein the body of plastic carrying the magnets is slit on one side to adapt the body to be pushed onto a tube neck from the side.

3. A device as defined in claim 2 wherein said body of plastic is provided with a recess on the side facing away from the slit for achieving a spring effect in the body.

4. A magnetic focussing device for a CRT having a screen and a neck for elongating the dots of light provided when the electron beam bombards the screen, said device comprising means defining two spaced magnet systems having magnetic fields arranged at other than right angles with respect to the electron beam of the CRT, pole shoes arranged so that when the device is mounted on a tube the shoes are directed toward. the tube neck, said pole shoes each extending over only a small portion of the circumference of the neck of such tube to form within the tube neck a magnetic field which is r-adially unsymmetrical and which is substantially parallel to the electron beam and to elongate the luminous spots on the screen in a direction which is at right angles to the screen lines, said means including a single annularly wound coil surrounded on all sides by a hollow core of magnetically conductive material, said core having a central opening having a diameter corresponding to that of a tube neck for surrounding the neck of a tube and including an air gap extending generally about the periphery of said opening, said magnet systems being electromagnets defined by the air gap at two diametrically spaced locations being sufliciently narrow in comparison to other locations being relatively large to define the shoes and provide substantially stronger magnetic fields in the vicinity where the gap is narrow.

5. A device asdefined in claim 4 comprising resilient tongues of an insulating material mounted in said opening and oriented in axial direction for firmly fitting against the tube neck for guaranteeing a secure mounting of the core on the tube neck.

6. A device as defined in claim 4 wherein the air gap in the inner opening extends along an edge of the opening.

7. A device as defined in claim 4 wherein said air gap is disposed in the center of said opening.

8. A magnetic focussing device for a CRT having a screen and a neck for elongating the dots of light provided when the electron beam bombards the screen, said device comprising means defining two spaced magnet systems having magnetic fields arranged at other than right angles with respect to the electron beam of the CRT, pole shoes arranged so that when the device is mounted on a tube the shoes are directed toward the tube neck, said means including a single annularly wound coil surrounded on all sides by a hollow core of magnetically conductive material, said core having a central opening having a diameter corresponding to that of a tube neck for surrounding the neck of -a tube and including an air gap extending generally about the periphery of said opening, said magnet systems being electromagnets defined by the air gap at two diametrically spaced locations being sufliciently narrow in comparison to other locations being relatively large to provide substantially stronger magnetic fields in the vicinity where the gap is narrow, the air gap running spirally between the two places with broad air gap from one side to the other side of the core.

9. A magnetic focussing system for a CRT for suppressing the line structure of a picture raster comprising means defining two spaced magnet systems having magnetic fields arranged at other than right angles with respect to the electron beam of the CRT, pole shoes arranged so that when the device is mounted on a tube the shoes are directed toward the tube neck, said means including a single annula-rly wound coil surrounded on all sides by a hollow core of magnetically conductive material, said core having a central opening having a diameter corresponding to that of a tube neck for surrounding the neck of a tube and including an air gap extending generally about the periphery of said opening, said magnet systems being electromagnets defined by the air gap at two diametrically spaced locations being suffic'iently narrow in comparison to other locations being relatively large to provide substantially stronger magnetic fields in the vicinity where the gap is narrow, and a circuit for energizing said coil and thus said electromagnets and including a first switch arranged to be mounted externally of a receiver in which the CRT is used.

10. A device as defined in claim 9 wherein said circuit includes a second switch coupled with the first switch, a focussing electrode for the CRT and controlled by said second switch to be connected to .a potential provided by said circuit which is such that optimum focussing is accomplished both when the device for suppressing the line structure is switched in, as well as when this device is switched off.

11. A device as defined in claim 9 wherein said first;

switch includes a switch contact connected into said circuit for controlling the picture tube to be dark for a short period of time during operation of the switch.

References Cited by the Examiner UNITED STATES PATENTS 2,648,036 8/1953 Walton 3l38.45 2,718,606 9/ 1955 Zuerker 31384.5 2,784,342 3 1957 Van Overbeek 31530 2,883,569 4/1959 Kaiser et a1. 31384.5

DAVID G. REDIN-BAUGH, Primary Examiner. 

1. A MAGNETIC FOCUSSING DEVICE FOR A CRT HAVING A SCREEN AND A NECK FOR ELONGATING THE DOTS OF LIGHT PROVIDED WHEN THE ELECTRON BEAM BOMBARDS THE SCREEN, SAID DEVICE COMPRISING A BODY OF PLASTIC MATERIAL ARRANGED TO BE MOUNTED ON THE NECK OF A CRT, TWO MAGNET SYSTEMS MOUNTED IN SAID BODY AND ARRANGED TO BE DIAMETRICALLY OPPOSITE EACH OTHER WITH RESPECT TO A TUBE NECK WITH THE MAGNETIC FIELDS THEREOF DISPOSED IN A POSITION WHICH IS OTHER THAN AT RIGHT ANGLES WITH RESPECT TO THE ELECTRON BEAM OF THE TUBE, SAID MAGNET SYSTEMS HAVING POLE SHOES ARRANGED SO THAT WHEN THE DEVICE IS ON A TUBE THE SHOES ARE DIRECTED TOWARD THE TUBE NECK, SAID POLE SHOES EACH EXTENDING OVER ONLY A SMALL PORTION OF THE CIRCUMFERENCE OF THE NECK OF SUCH TUBE TO FORM WITHIN THE TUBE NECK A MAGNETIC FIELD WHICH IS RADIALLY UNSYMMETRICAL AND WHICH IS SUBSTANTIALLY PARALLEL TO THE ELECTRON BEAM AND TO ELONGATE THE LUMINOUS SPOTS ON THE SCREEN IN A DIRECTION WHICH IS AT RIGHT ANGLES TO THE SCREEN LINES. 